[SDAG] propagate FMF from target-specific IR intrinsics

This is a step towards relying more on node-level FMF rather than function-wide
or target settings.
I think it was just an oversight that we didn't get this path in D87361
or follow-on patches.

The lack of FMF propagation is blocking D90901 from converting tests to IR-level FMF.

We can't do much more than this currently because we also fail to propagate flags
from x86-specific node to generic FMA node. That would be another patch, so the
test just verifies that we can transfer from IR to initial SDAG node.

Differential Revision: https://reviews.llvm.org/D102725